[ntp:questions] NTP sync problem. "no server reachable"

Dave Hart davehart at gmail.com
Mon Mar 9 23:57:13 UTC 2009


On Mar 9, 5:00 am, kiran.shi... at gmail.com (Kiran Shirol) wrote:
> If that is the case, I should not be getting any responses ?Or  are you
> saying NTP gets the responses and then drops them ?

NTP gets the responses, finds the time too far apart on the two
sources, has no way too decide which one to believe, and so becomes
unsynchronized.  It continues to trim the local clock based on the
observed frequency drift but will not sync to either server until
their times get close enough together to be believable, or one becomes
unreachable.

Short version:  Never configure only two sources.

Keep in mind that with only three sources, the unreachability of any
one could leave you in the same situation if the remaining two are not
sufficiently in sync with each other.

For meatier detail see:

http://www.eecis.udel.edu/~mills/ntp.html

In particular see page 15 of:

http://www.eecis.udel.edu/~mills/database/brief/arch/arch.pdf

Your situation is akin to having only sources C and D in that diagram
available.  Since there is no overlap in the confidence intervals,
there's no way to know which to believe.  In case it's transient (such
as temporary unreachability) NTP continues on without a time source,
"freewheeling."

Cheers,
Dave Hart




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